Mobile is an extremely important target for Apple. The carrier is the largest in the world, with over 700 million subscribers. While most of them likely can't afford an iPhone, reaching even a fraction of that customer base could prove a major boost to iPhone sales. The core obstacle has been Mobile's decision to go with a proprietary 3G data standard, TD-SCDMA. The iPhone 5s and 5c both support Mobile's TD-LTE 4G standard, however, leaving no technical obstacles to adoption apart from the reach of the carrier's 4G network.